Fix buggy key handling in CompositeAnimation structures.

Stale map keys are left in the order vector.

Plus, tidy up some whitespace, and fix a bad name (implying a Vector was a Map).

Also, move away from AtomicStringImpl which we're getting rid of in bug 250050. Instead of using pointers to a StringImpl, which is fragile, we just use AtomicString directly and atomize the name earlier in the CSSAnimationData object.

Testing is covered by existing tests plus adding an ASSERT and tightening an
invariant that keys in the order list must not be stale. The ASSERT fires with
existing tests.

BUG=256794
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/18573002

git-svn-id: svn://svn.chromium.org/blink/trunk@154705 bbb929c8-8fbe-4397-9dbb-9b2b20218538